Stress-aging process



Patented Apr. 2 1 1951 UNITED STATES signer to General Electric Company, a corpora tion of New York This application is a continuation-impart of my copendingapplication Serial No. 663,754, filed April20, 1946, now abandoned, and'assigned to the same assignee as the present invention.

The present invention is a heat treatment combined with an applied stress, for alloys and particularly alloys which are constituted either wholly or principally of one or more solid solutionphass. It is one of the objects of the present invention to provide a method for improving the properties of such alloys and particularly the elastic properties and electrical conductivity. Other objects will appear hereinafter.

The novel treatment of the present invention broadly comprises heating alloys, constituted wholly or in major part of a solid solution phase (or phases) for correct times at effective elevated temperatures while the material is subjected to efiective stress from an externally applied load. The resulting effects on physical properties, compared with all past experience for any of the previously known treatments, are startling, to say the least. By means of this new treatment it is possible in certain alloys (although all such alloys will be advantageously aiiected to some degree), to double and almost triple the elastic properties together with an increased tensile strength combined with a previously unknown simultaneous increase in elongation and increase "in electrical conductivity. Considering tensile properties alone, it has heretofore been con- "sidered impossible to effect such increases in strength properties of solid solution alloys without drastic cold reduction whereas the process of the present invention does not involve any measurable plastic deformation. Also it has been previously considered impossible to obtain such increased tensile properties without coincident decrease in elongation by any known treatment, whether by cold working or by heat treatment.

Prior tothe present invention, hereinafter designated a stress-aging process, all teaching and N Drawing. Application September 30, 1948, Serial No. 52,145

Claims. (Cl. 14811.5)

experience have indicated that single phase-solid solution alloys could not be given any heat treatment other than a strain relief of the cold worked materials, or an anneal, and-that the only way to strengthen solid solution alloys was by cold work. The prior art teaching and experience also has indicated that such cold worked materials could not be further strengthened'with out further cold reduction and that all additional cold reduction was progressively less effective. In my improved process which comprises 'a heat treatment under applied external stress, itwill be shown that such prior art teachings are 'erroneous and that heat treating or aging under applied external stress effects Ia great improvement in alloy properties.

The present process comprises heating alloys, consisting wholly or in major part of onelor more solid solution phases, at temperatures below .the recrystallization temperature but high enough to provide maximum diffusion While stillgproviding a condition of elasticity in the crystal lattice,

while the alloy is submitted to' externally applied tension (or compression) loads of aboutmaximum elastic value. Above'the recrystallization temperature, such alloys are practically com pletely lacking in elasticity, are-readily p'l'asti cally deformed (or hot worked) and have -'rapid rates of creep. even for the'slightestloads. The loads chosen for stress-aging efiects of maxi' mum degree are the stresses "in the jrangeof the proportional limit, and maybe ashighjais the 0.2% offset yield strength, determinedfior the alloy in its condition immediately previou'sj to stress aging, so long assaid loads do not rate measurable (destructive) creep during th eStress aging period. Of course, any processes depend}- ing upon diffusion involve time, and, as will be shown hereinafter, time at stress-aging temper ature is a factor and will vary with difierentalloy compositions. Since stress is always measured in terms of the load per unit area, the new process is correctly named stress-aging. However,

due to common improper confusion of the two terms stress and strain, such a less useful name as load-aging might avoid confusion of stress-aging? with stress-relief, etc.

The similarity of nomenclature between stress-relief and stress-aging may be confusing solely on'lthe basis of the language employed. However, in view of the established and present definitions, they are readily seen to be distinctively different processes for distinctively different purposes and with distinctively different property effects.

Cold worked or cast alloys are usually characterized by internal elastic strains that may cause warping after machining, etc. Strain relief (or stress relief consists of heating the alloy to relatively low'temperatures to relieve the crystal lattice of elastic distortion. In the case of coldworked alloys, the object is to relieve this elastic lattice distortion with as little softening eifect as possible, retaining as much as possible the high strength properties of the plain cold-worked condition. The tendency of strain relief treatment is to lower the strength properties and to increase the elongation. Annealing, on the other hand, consists of' heating to a temperature above the recrystallization range, chiefly in order to remove all property and structure effects of previous cold work. Annealing results in minimum strength properties and maximum elongation.

The present invention, in general, may be applied to all alloys which comprise in whole or major part one or more solid solution phases. Qualitatively, rates of diffusion in metals and alloys at room temperature are proportional to the specific melting points or melting ranges, and thermal agitation of atoms within the crystal structure is proportional to temperature. Metals and alloys with higher melting points and ranges have higher recrystallization temperatures and aremore stable at room temperature. Thus, cold worked alloys of aluminum, copper, nickel, iron, etc. maintain their cold worked structures at .room temperature and up to higher temperatures In carrying out the present invention, alloys constituted wholly or principally of one or more vsolid solution phases are heated generally at temperatures well above 100 C., while external stress is applied to the alloy. The most effective stress-aging temperatures are within the range extending from somewhat below and up to the temperatures employed for softening in standard annealing, or, in other words, extends from about 100 C. below and up to the recrystallization temperature. The temperatures employed and the results obtained distinguish the present process from the phenomenon known as strainaging. This is a phenomenon not yet truly understood. The efiects of this phenomenon have been measured and are clearly described in Metals, Carpenter and Robertson, vol. I, 1939, Oxford University Press, N. Y. C., page 174, as follows: In general, the limit of proportionality, yield-point, and ultimate tensile strength should decrease as the temperature of testing is raised.

Thus, alloys I So far as the first two are concerned this statement is approximately true, but it is far from true when applied to the ultimate tensile strength, when the metal under consideration undergoes substantial strain-aging. Thus strain-aging is some kind of strengthening efiect taking place at stresses (applied loads) high in the permanent deformation range, above the yield-point, during a tensile test at an elevated temperature within a rather critical range of temperature, which, in turn, is not close to or at all associated with the recrystallization range. Furthermore, the increased tensile strength is accompanied by a sharp decrease in elongation. Strain-aging is pronounced in iron and steels with a maximum strengthening effect at about 250 C. and is not related to the recrystallization range (450600 0.). In most'non-ferrous alloys strain-aging does not exist, A very much smaller bulge in the tensile-strength vs. testing-temperature curve for some non-ferrous alloys cannot be definitely associated with strain-aging in steel.

Applicants process of stress-aging is carried on under constant external loads, equivalent to the proportional limit-0.2% offset yield strength range of the alloys previous room temperature stress-strain curve, said loads being applied for extended periods of time at temperatures just below the alloys recrystallization temperature, said stress-aging process being markedly effective on both ferrous and non-ferrous alloys, alike, in markedly increasing the subsequent room-temperature, proportional limit and yield strength of the stress-aging alloy. Thus the phenomena and results of applicants stress-aging are obviously entirely different from those of strainaging.

The beneficial effect of the stress-aging process increases with increase in the applied stress, increase in aging temperature (until a temperature is reached beyond which the tendency to anneal overcomes'the stress-aging reaction), and increase in the time at temperature of aging. A twenty hour aging period usually provides satisfactory results although alloys with very slow diffusion rates respond to longer aging periods. In general, an aging period of four hours achieves about to'95% of the complete effect of stressaging and in many cases an aging period of onehalf hour, or even 15 minutes, will accomplish useful results.

The most effective stresses which may be employed with any alloy having a solid solution dominated structure are those which range from the proportional limit to the 0.2% off-set yield strength determined for the material in the condition preceding stress-aging. In certain cases wherein the coincident creep or flow is not appreciable still higher stresses will be found to be beneficial. Stresses lower than the proportional limit of the initial condition of the alloy are much less effective and in many cases would not justify a special treatment. Otherwise defined, for any given temperature within the prescribed range hereinbefore given, the stress should equal or exceed the proportional limit of the alloy (before stress-aging) but should not be great enough to efiect a practically measurable permanent deformation at the stress-aging temperature, i. e. should not effect a permanent deformation greater than about 1 mil per inch. Alloys with higher melting ranges require greater applied stresses for maximum stress aging re- 4 sults. For example, although stresses as low as 20,000 to 25,000 pounds per square inch are effective for magnesium and. aluminum alloys stresses far in excess of 50,000 to 60,000 pounds per square inch may be required for iron, nickel and cobalt alloys.

The proportional limit and offset yield strengths referred to in the description of the present invention are co-related properties derived from the stress-strain curves for standard tensile tests of metals and alloys. The firstjpart of the stress-strain curve for those alloys whose recrystallization temperature is above room temperature, approximates a straight line and indicates that, throughout its range, the strain is directly proportional to the applied stress. The

sample), and drawing a line from this point, parallel to the proportionality tangent at the proportional limit, to intersection with the stressstrain curve. Thus the 0.1% offset and 0.2% offset yield strengths are determined and are corelated to the proportional limit. Tests have shown (as is well-known) that the elastic limit may be as low as the proportional limit, but, in the vast majority of cases, elastic behavior extends above the proportional limit and may even extend beyond the 0.2% offset yield strength. Therefore, in the basic definition of stress'aging, the applied elastic stresses to effect maximum stress-aging effects are stated to be in the range of the proportional limit and may be as high as the 0.2% offset yield strength. Generally, loads in excess of the 0.2% offset yield strength cause measurable (true) permanent (plastic) deformation. Only the elastic portion of a plastic deformation load is effective in stress-aging and, since stress-aging is to be applied to stock (such as-plate; rod, strip and Wire) or shaped parts already finished to desired dimensions with close tolerances any further plastic deformation during stress-aging would be damaging or destructive to the 'material or parts thereof. Therefore, it is desirable that the stresses for stress-aging shall not be high enough to cause any damaging or destructive creep (permanent extension or distortion) during the stress-aging cycle.

Thebeneficial effects of my stress-aging process improve with increasing solute metal content. However, relatively small quantities of an alloy ingredient, whether an intentional addition or a normal impurity, may be very effective as in an alloy containing 1% cadmium and 99% copper. The ferrite matrix of plain carbon steels contain varying small quantities of carbon, manganese, silicon, etc. and such material has capacity for stress-aging. The same condition applies to commercial alloys all of which possess some degree of stress ageability. The metals, however, and particularly pure metals which lack any alloying ingredients, are so soft and characterized by such low elastic properties that stress-aging is far less effective than when applied to alloys. In general, therefore, my stressaging process will be employed to best advantage only with alloys.

The stress-aging process may be carried out in various ways, for example formed springs of wire may be compressed, extended, or deflected and the stressed spring aged while heated to the proper temperature by highfrequency current or any other suitable means. Also, strip or wire may be stressed in tension and a ed while heated by electrical resistance methods or the like. As applied to formed springs of wire, the process should not be-confused with certain prior processes such as stabilizing If a sheet material of an alloy possessing good formability (such as aluminum alloys Alcoa 52S, 518T, 53ST, GIST,

- etc.) be cold formed into a desired shape for any spring-like use, repeated flexings in use will cause a re-alignment of the elastic strains (from cold forming) with a resultant warping of the part. If, after forming, the springing member be placed in a jig or form'so that said membermay be kept under slight elastic deflection in the desired operating direction, and said member (so restricted elastically) be heated at only slightly elevated temperatures (preferably at F. or 65 C.) and re-cooled to room temperature under the same elastic restraint, the elastic strains will be re-aligned geometrically with the direction of spring operation during subsequent use of the part and no warpin will occur during use. The standard tensile properties remain practically unaffected by this stabilizing treatment.

While this process of stabilizing is mechanically similar, in steps performed, to the steps employed in applicants stress-aging process, both the applied stress in stabilizing and the temperature employed for stabilizing are too low to effect the marked increases in properties developed by applicants stress-aging. In fact, one desired purpose of stabilizing is to retain as nearly as possible the original properties of the sheet material on which is predicated (by its engineering design) the operation in use of the stabilized part. Due consideration shows that stabilizing is thus distinctly different from applicants stress-aging. For example, applicants data show that stress-aging is ineffective for alloy 52S at temperatures below 125 0. (257 F.) and the benefits for stress-aging alloy 52S reach a maximum at C. (347 F.). Moreover, the (elastic) stabilizing treatment is effective only for cold-worked (formed) parts, inclusive of pure metals, whereas applicants stress-aging is shown to be effective for annealed alloys as well, but only for alloys containing effective amounts of solute metals (in solid solution).

Another example of stabilizing treatment is one applied to any spring acting part out from cold-rolled sheet or strip. Thus coiled springs of cold-rolled brass, cold rolled steel, or of himetal (duplex strip of two alloys bonded together), etc., are coiled and tied in the coil shape (under slight elastic strain) and, so coiled, are stabilized at as low an elevated temperature as possible, depending on the alloy compositions involved.

Strand-annealing is an established method of high efficiency and economy for continuously annealing. or softening cold worked strip and wire. The strip or wire is drawn from loose coils or very free running reels or spools, pulled through the annealing furnace and rewound at the furnace exit. Since the purpose of strand annealing is to produce practically dead soft strip and wire for subsequent forming or further cold reduction, the strand-annealed product has very low strength properties. In annealing such soft materials, the stress employed in pulling the free runningstrip or wire through the furnace is kept as low as possible and far too low for any stress- 7 aging effects since practical annea -ling temperatures are considerably in excess of the recrystallization temperature and therefore too high for stress-aging. However, if the conventional strand annealing equipment is altered to maintain a desired high stress and proper relatively low aging temperatures, the desired stress aging properties maybe developed for the chosen material. This application of the invention differs from various means for controlling strand anneali-ng temperatures which involve controlled plastic deformation and make use of the principle that each metal and alloy possess a, definite tensile strength for each temperature so that tensile strength, may be used to measure temperature. lit-is well known, or course, that this relationship is true only for temperatures above the recrystallization temperature (or above the age-hardening temperature), as below these temperatures the tensile strength of any alloy will vary with the amount of preceding cold work (or degree of preceding precipitation hardening). Thus the direct relationship of tensile strength to temperature applies only throughout the true hotworking range. In the hot-working range of temperatures, all metals and alloysa're completely lacking in elasticity and hence possess no proportional limit, no elastic limit and none of the related ofiset yield strengths and any effective applied stress (below the tensile strength) is a plastic yield strength causing continuous yielding at rates varying with the amount of the applied stress and the temperature: the rate of yielding being approximately constant for a maintained constant stress and constant temperature. For strand-annealing and for strand-solution treatments (both at temperatures well above the respective recrystallization and ageha'rdening temperatures), wire or strip is passed continuously through a zone at the desired controlled elevated temperature. The length of this zone is necessarily related to the temperature and the time required at this temperature to achieve the necessary annealing or solution treatment for the specific alloy. When the correct relationship of high temperature, rate or" travel of the strip or wire, and the length of the hot zone have been determined, it only remains in practice to control the temperature. Since, in the range or" hot working temperatures, the tensile strength of the metal or alloy at the desired temperature is closely related to said temperature, a suitably lower stress may be applied to the strip or wire (by various mechanical means. as the strip or wire is passed through the estbalished hot zone. Such a plastic yield strength, effective for the previously fixed length of the zone at temperature, will cause a specific amount of elongation and reduction in area of the strip and wire. Thus, under the given design and operating conditions for the hot zone, the rate of yield or amount of elongation (if measured continuously) or the reduced cross-section (of the strip or wire) can be used in various ways to effect tempera ture control (as by regulating fuel combustion or electric power for resistance heating). To employ such method of temperature control, it is usually necessary to have the wire or strip at a suitable oversize previous to the heat treatment, depending upon the desired'tolerances for the finished product It is obvious that processes employing either plastic deformation or temperatures resulting in recrystallization or over-aging (or solution) preclude the use of applicants process of stress ag'- ing. Moreover, the elastic stresses required for applicants stress-aging process far exceed the plastic yield strengths employed in such processes, in fact, in most cases, would exceed the tensile strengths of the alloys at the operating temperatures of such processes.

While stress-aging, the new treatment, consists of heating alloys under externally applied loads (or stresses) at temperatures not in ex cess of the recrystallization range, it is to be noted that none of the previously known heat treatments are conducted on materials under stress from external loads, for example, under tension. The following table summarizes pictorially the effects of these known treatments on physical properties as compared to the new treatment: Stress-aging.

l increase, D decrease, C constant,

A The use of parenthesis indicates alternative cllect.

The efiect of my stress-aging treatment can be made additive to the properties for cold worked solid solutions and also additive for aging of precipitation hardening. It also may be combined with or superimposed upon the tempering and annealing treatments for steels since the matrix for all steel microstructures consists of either alpha or gamma solid solutions based on the lattices oi alpha or gamma iron.

'-lhe benefits of the stress-aging process as applied to each of eight commercial copper base alloys are hereinafter set forth in lables Nos.

to 5 inclusive. The bars or test specimens were standard types, (a) threaded ends with 0.350" diameter reduced section in excess of 2 gage length and (b) threaded ends with 0.250 diameter reduced section in excess of the 2 gage length. Size (at) bars were use-d for all alloys except the initially higher strength 10 ,1. tin-Phosphor bronze for which size (1)) was used for testing convenience. In each case a stress usually within the range of values between the proportional limit and a chosen yield strength relative to the specific alloy, was applied. Wl-iile under such stress the alloys were heated for a desired length of time -at effective temperatures and then cooled to room temperature under maintained stress. lneach table the (A) lines indicate the property of the alloy in the stated commercial condition previous to stress-aging. All values in the tables are average for two or more bars each treatedindividually. The aging temperatures in all tables were chosen in the range of initial softening when the cold worked material is simply reheated as for standard strain relief and annealing. Heating periods of 4 and 20 hours were chosen arbitrarily;

TABLE 1 Stress-Aging 1 Prop 2 1 Tensile El Line Limit, %Y1ed Strength, Percent,

Stress, S i. Strength i Per Cent R. A. H13. 0.

70-30 BRASS: 30% GOLD REDUCED 41.... none 23,500 54,450 09,150 17 34 23. 500 4 175 33,000 02,250 70,700 19.5 30 40, 000 4 175 41,900 01,400 71,200 18 73 54,100 4 175 54,300 00,500 71,750 21 75 4-.... 54,100 20 175 50,500 00,700 ,700 71 5 40,000 4 200 45,200 05,000 71,300 18 71 0-.... 54,000 4 200 50,300 00,500 70,000 49 70-30 BRASS: 52% COLD REDUCED 42.... none 4 23,400 09,300 30,750 13 3..... 23,400 4 175 39,200 74,450 87,250 115 75 9-..-. 40, 000 4 175 49,000 30,000 88,500 115 73 10..-- 54,100 4 175 .00, 400 35,400 88,700 12 73 11.... 09,800 20 175 71,000 80,300 33, 750 10 71 f 12..-. 40, 000 4 200 55,200 .37, 400 00,000 11 71 1 13.... 54,000 4 200 00,200 37,400 39,300 11 71 14.... 54, 000 4 225 88,000 83,500 10 01 TABLE 2 Str ess-Aging P l T 1 I 4 2'09, 027 Yield E1011"; P01100111; l V i Llne Lnmt, 0 Strength,

StressY Hr 0 p.51. Strength p.51. P0100115 R.A. 1 p. s. i. 4 j

I 85Cu13.25Zn-1.75Pb: GOLD REDUCED 113.... none 23, 000 00,300 01, 000 10 00 54,000 4 175 55,300 00,400 00,400 10 00 40, 000 4 200 44,000 00,000 00,300 11 00 54, 000 4 200 53,400 53,300 53,300 10 00 4-.-.- 40,000 4 225 42,500 53,750 59,400 12 00 5. 54, 000 4 225 bro 1:0 in furnace3 hrs. 7 48 0.---. 0,000 4 250 44,100 57,300 53,350 12.5 00 ,000 4 275 42,350 53,700 55,050 10 48 85Cu13.25Zn-1.75Pb: COLD REDUCED 44.... none 27,350 75, 000 70,200 3.5 00 3.-.-. 54, 000 4 175 00, 000 75,000 75,700 0 00 9-.... 40, 000 4 200 51,900 74,300 75,500 9 00 10--.. 03,500 4 200 00,900 74,000 75,000 10 00 11.... 40, 000 4 225 49,000 72,350 74, 500 10 00 12--.. 03,50 4 225 03,100 71,900 72,750 9.5 43 13---- 40, 000 4 250 43,400 71,500 72,000 9 00 40, 000 4 275 50,300 09,300 71,250 10 TABLE 3 Stress-Aging P l r I F 0 27 Yield e Elong Per Cent Lme L1m113, Strength,

Stress, Hrs. OC- p. S 1 Strength p. PerCent RA. p. s. 1.

5sn950u: 00% COLD REDUCED 10311-90011: 00% COLD REDUCED TABLE 4 Stress Aging P l P l in? 027 Yield Elonr'. PerCent Line Limit, 9 Strength,

stress, c p L Shength p.51 .PcrCrnt R. A. V H15. 0. p. s. 1

1Cd-09Cu:60% COLD REDUCED none 32,150 00, 800 70,100- 10- 70 32,150 4 175 53,050 72, 350 72,350 1.1 74 40,000 4 175 40,500 71,100 71,100 11 73 04,800 4 175 57,100 70,550 70,550 9.5 71 40,000 175 53,100 70, 000 70,000 10 71 40,000 4 200 57, 750 72,000 72,900 9.4 09 04,800 4 200 05,000 71, 000 71, 000 3.0 08 04,800 4 225 00, 700 71,800 71, 000 12.5 08

TABLE 5 Stress Aging P 1 1 T 8 my? 0.27 Yield "B11510" Elong Per Cent LlIlO Llnnt, Strength,

stress Hrs. c C- p S i Strength p. L Per Cent. R. A. p. S. 1.

8.3A12.9Fe0.30Ni-88.5Cu:HOl EXTRUDED A8 none 25,000 32, 050 1 81,100 40 00 25,000 4 175 20,000 32,100 30, 500 42 02 2,... 32,000 4 175 34, 700 30,200 00, 000 35 00 32,000 4 200 32,250 34,200 80,200 01 4 32,000 20 200 34,000 30,200 81, 000- 54 In Table #1 the proportional limit stress 28,500 sile properties from those given in line A3. In per square inch applied to the 36% cold reduced lines numbered 6 and 7 the test results show a alloy while aging for four hours at 175 C. gave 35 slight lowering of tensile strength and yield increases of 18% for proportional limit,"1'4%' strength from the stress aging maxima for 40,000 for 0.2% yield strength, 2% for tensile strength, pounds per square. inch (line numbered 2,.Table and 15% for elongation, whereas the yield 2) due to opposition of the annealing tendency strength stress 54,000 per square inch gave reat higher temperatures. A, stress of 54,000 spective increases of 90%, 18%, 4% and 24%. 10 pounds per square inch actually caused embrittle- A comparison of the results set forth on the line inent and fracture of the test bar during treat numbered 3 with those set forth on the line ment at 225 C. The free lead in the hardware numbered 4 shows that four hours aging gives bronze alloy largely nullifiesany stress aging efabout 96% of the increase in elastic propertiesfectsonv elongation and reduction of area so that ii. e. proportional limit, elastic limit and yield in the absence of any embrittlement for excessive strengthoffset from 0.2% to 0.5% from the proaging under stress these two properties remain portionality range of the stress strained curve) relatively constant. accomplished by 20 hours aging. The reduced alloy set forth in Table #2 In the test on "1'030 brass cold reduced 52% stressages with surprising improvement in propit will be noted that the increase in cold working 5 erties at higher temperatures that, in the absence provides a marked increase in yield strength-and of stress, would have an annealing effect. The tensile strength with decreased elongation therepresence of lead in hardware bronze tends to perby permitting the use of higher stresses for stressmit plastic flow: once: plastic deformation is well aging without damage from creep effects. started. Hence, stress-aging has relatively small Table #2 sets forth the test results on 'hard- 5 effect on elongation and reduction of area and ware bronze, cold reduced 35% and 50%. As the fractures retain the cup and cone type of is normal for solid solution alloys the lower solute breakv throughout the series. content (13.25% zinc) of hardware bronze as Tab1e'#3 discloses the result of tests on two compared with 30% zinc in the '7030 brass retin-Phosphor bronze alloys each cold re sults in lower elastic properties and lower tensile duced and. containing respectively 5 and 10% tin.

strength. However, contrary to the usual. ef: fect of decreased solute metal content, clecreasing the zinc content in copper zinc alloys actually results in a" decrease in elongation. This increased basic rigidity causes a higher 0.2% yield strength in the lower zinc alloy, the yield strength being actually at or within the range of marked plastic deformation. This anomalous efieet isapparently due to the unusual nature of the zinc atom, relative to the copper atom in the solidsclution lattices. As indicated in lines. numbered 4, 5, 6 and '7 of Table #2, the stress aging is surprisingly effective at those high temperatures which in the absence of stress actually exert an annealing effect causing a depreciation inten:

The. phosphorus. content. of each. alloy is about 0.1% remaining after deoxidation. The stress aging effects for these alloys are of the same order as found for the other alloys of this series which lack phosphorus. The results obtained therefore are apparently not due' to the small quantity of phosphorus present in the tin-Phosphor bronze alloys.

-ZLportionallimit, 10% in the 0.1% yield strength 7 Tables 1 to 4 inclusive;

l3. 1 (64,800 per square inch). 3% in tensile strength and 10%in elongation.

Table #5 gives the properties of .hot extruded aluminum bronze containing, 8.3% aluminum,

, 2.9% ,iron, 0.30% nickel with the balance copper. This. is a standard single phase commercial aluminum bronze composition. The alloy was hot'. extruded at temperatures well above the anneal ing temperature so that no cold Work was ef-- fected. The tensile properties of the material untreated and as received are set forth in line A8 and its microstructure shows the alloy to consist of a single solid solution phase free fromany cold Work.

The data for the alloys set forth in Tables 1 to 4' inclusive show that stresses in excess of stresses duringthe aging periods.

For each of the eight alloys hereinbefore set forth the efiects on electrical conductivity of two of the most eifective stress-aging treatments are given in the following table:

- TABLE Efiect of stress-aging treatments on electrical conductivity I E1 Stress-Aging E1 3C. g 96. Line Alloy Condy., Condyz, i Per Cent Stress, Hrs o 0 Percent 1X... 70-30 Brass 21 0 40,000 4 200 27.5 27 M 30% e010 Work 21.0 54,000 4 200 27.3 20

70-30 Brass. 21 54,000 4 175 7.1 30 52% 0010 Work... 21 04,000 4 225 28.4 30

Hdware Bronze... 29.0 04,000 4 200 32.0 10 32 30% Cold Work... 20.0 40,000 4 275 34.3 19

4X.... Hdware 13101120.. 20.2 03,500 4 200 35.3 21 42...... 50% 00111 Work... 29.2 40,000 4 270 35.4 21

0X Sn P-Br0nze. 10.3 75,000 4 170 20. 7 27 52 00% Cold Work... 10.3 ,000 4 225 21.7 33

0X..... Sn P-l3ronze 8.2 I 114,000 4 175 10.4 27 02 00% Cold Work... 8.2 114,000 4 220 10.5

7X...- 1 o 199 Cu 09.4 40,000 4 200 83.0 20 7z 00% Cold Work... 09.4 04,000 4 200 33.1 20

Al-Bronze 11.9 32,000 4 200 12.1 .2 82 No Gold W0rk 11.9 32,000 20 200 12.1 2

about 30,000 pounds per square inch produce the great improvement that can be effected by stress aging. in the annealed condition all of the copper base alloys set forth in Tables 1 to 4 inclusive lack the necessary strength to support the stresses required for the most effective stressaging. The properties of aluminum bronze with a proportional limit of 25,000 pounds per square inch indicates that the mere addition of a cold worked structure is not a requirement for stressaging other than for initial strengthening purposes. However, due to its relatively high elongation (40% in the hot extruded or annealed condition) the 0.2% yield strength of about 33,- 000 pounds per squareinch is much lower than for any of the cold worked alloys set forth in.

Thus, only a very narrow range of aging stresses, between 25,000 pounds per square inch and 33,000 pounds per square inch, can be expected to be really effective for stress aging this aluminum bronze alloy since higher loads would cause destructive plastic deformation within the time required for The data in this table indicates,

(1) that stress aging increases electrical conductivity in all cases, (2) that for the same aging treatment stress, (5) that with sufficiently effective stressaging, the resulting electrical conductivities may even exceed that of the annealed single-phase solid solution alloy, as indicated in the following items:

(12) Electrical conductivity of annealed -30 brass is 27% of that of pure copper. The resulting conductivitiesfor 1X, 12, 2X and 2Z are slightly in excess of 27%.

, (b) Electric conductivity of annealed 5Sn bronze effective stress-aging at any useful temperature. 7

This aluminum bronze will not support a stress of 32,000 pounds per square inch for much more than four hours at 200 C. or as long as four I is 17%. Stress aged bars 5X and 5Z have conductivities about 20 to 30% in excess of 17% (0) Electrical conductivity of annealed 10Snbronze .is about 9%. Stress aged bars SK and SZ; have conductivities about l5% in excess of 9%. r

5 (Class A), u

15 other solid solution alloys such as the following can be processed by the present invention:

Class B.Stainless steel Class C'.Solid solution Fe-base alloys Class D.Precipitation-hardened Fe-base alloy Class E.Solid solution Al-base alloys Class F.Precipitation-hardened (or heat-treated) Al-base alloys.

It. is to be understood of course that this classification is wholly arbitrary .and non-limiting.

CLASS B.STAINLESS. STEEL The material treated had the following typical analysis: 18.33% Cr9.32% Ni0.10% C0.91% Mn0.41% Si and the balance. Fe. The stock was /2" diameter hard drawn rod, finished with 38% cold reduction, giving, about the maximum strength properties for rod of this size.

Reference to Tables 7 and 7A will give the results of stress-a ing the stainless steel. Line 1 in both tables gives the tensile properties: of the plain cold worked alloy (as received) Stainless steel of this type isa rather complicated alloy (not an un-heat-treatable single solid-solution phase as is alpha brass). Stainless steel can be made austenitic at room temperature by quenching it from above 1000 C. Subsequent cold work tends to transform the Austenite into alpha ferrite. Also reheating in the range of 400-600 C. causes precipitation of carbides if even 0.1% C ispresent (unless the alloy is stabilized by converting the carbon into insoluble carbides, such asthose of tantalum or columbium).

TABLE 7 strated by line2, Table 7. Line 3 shows the coldwork strengthening of stainless. steel at room temperature caused by a stress of 90,000 p. s. i., just above the elastic limit. Line 4 shows the strengthening at room temperature by a stress of 150,000 p. s. i., well into the plastic range of this alloy.

Plastic deformation of an alloy at temperatures below its recrystallization range will cause cold work strengthening but this strengthening diminishes rapidly with increase in temperature of the cold-working. Thus the cold-work strengthening of alloys at temperatures just below their recrystallization range is greatly less than the strengthening by equal cold plastic deformation at room temperature, when room temperature is effectively below the recrystallization range. Thus the strengthening efiect of 90,000 p. s. i.

stress in the aging range of 400 to 600 C. is very much less than at room temperature. Therefore, comparison of line 3, Table 7, with lines 5 to 14 shows that the stress-aging effect on properties (proportional limit is of special importance for engineering design) of stainless steel cannot at all be explained by any cold-working interpretation. In fact,'the marked strengthening efiect of 90,000 p. s. i. stress-aging for only 15 minutes at; 400 C. (line 5) would rule out any efiective plastic flow for such a short time as 4 hrs. at these temperatures. There is no measurable creep until the stress-aging temperatures exceed 500. 0., whereupon the annealing efiect begins to overcome the stress-age-strengthening effect Properties of stress-aged 18 Cr-9. Ni stainless:

steel (cold worked 38%) Stress-Aging 7 Ofi 0.2 0, Elong.,

Line 53,, set Yield ffi j Per 7; f

Stress, Hrs 0C Strength g Cent p. S. 1

72, 300 178, 500 193, 200 7. 5 37 4 72, 200 178, 600 193, 800 7. 5 37 4 25 105,000 174,600 194,200 7.0 36 4 25 120, 000 176,600 192, 900 2.0 15 min. 400 133, 400 188, 900 202, 200 8. 5 36 30 min. 400 138, 700 187,700 202,600 8.5 38 min. 400 139, 700 188,200 201,900 8.5 38 1 400 138, 300 188, 600 202, 600 8. 5 38 2 400 140,000 188, 100 203,100 8. 5 38 4 400 142, 000 191,000 205, 400 25. 5 39 4 450 136, 000 197, 600 204, 000 8. 5 42 4 500 134, 000 193, 000 197, 300 8. 5 40 4 550 128, 000 170,000 182,000 6.0 46 4 600 114,000 159,000 176,500 13.5 42 4 400 154,000 206,000 208,800 8.5 42 4 450 Bar broke in furnace 4 500 Bar broke in furnace TABLE 7A Properties of plain aged 18. Cr-9 Ni. stainless steel (cold worked 38%) As is well known, stressing an alloy below its.

elastic limit for relatively short times (so that creep remains unmeasurable) will not affect its previous physical properties. The proportional and the stress-aged properties begin to decline (or over-age) Table 7A shows the effects on. tensile properties of aging the same alloy at the same temlimit is'below' the elastic limit. This is demonperatures (as for stress-aging) but in the absence I? of any externally applied stress. Lines 2 to 5 of Table 7A show relatively small' increases in strength properties due to the carbide precipitation effect previously described. This effect is distinctly minor as compared to the efiects of stress-aging stainless steel. It is also to be noted better than 90% of the improvement effected by 4 hours.

Stress-aging with 150,000 p. s. i. is effective on properties but destructive creep at this stress occurs at 400 C. and stress-agin at such higher stresses is impossible at higher temperatures as the alloy fractures during the treatment (shown by lines 15, 16, 17, Table 7) CLASS C.SOLID SOLUTION FeBASE ALLOYS These are solid-solution alloys that are not amenable to heat treatment except for annealing to remove all effect of cold work properties and structure. I

18 The compositions of these 3 representative al= loys were as follows:

The recrystallization and actual annealing amen-0.03p

temperatures are as follows:

Recrystalli- Annealing zatlon Tem- Temperapemture ture C. C. (A) 705 735 (B) 815 l, 040 (O) 815 1, 040

Since these alloys are in the annealed condition, simply reheating them to such lower temperatures as were employed for subsequent stressaging would have no measurable effect on their annealed properties. The properties of these alloys, in the initial annealed state, are given respectively in lines 1. '7 and 13 in Table 8.

TABLE 8 Properties of 3 annealed Fe-Base solid-solution alloys, Efiect of Stress-aging Stress-Aging Treatmm P 1 85 1 T 1 "131 r o I t rop se ensi e 0ng., er en Lme Creep Limit Yield Strength Per Cent R. A.

Stress, Hrs a C Strength p. s. 1.

A. 36% Ni64% Fe ALLOY, ANNEALED None 25, 000 40, 000 70, 000 65 25,000 4 400 0 45, 800 68; 000 84, 500 21 81 40, 000 4 400 0 56, 500 69, 500 84, 500 21 81 000 20 400 001 57, 200 65, 400 81, 200 20 80 25, 000 4 500 005 47, 800 59, S00 80, 700 25 78 40, 000 4 500 300 52, 600 68, 800 84, 800 16 77 B. l9Ni-2Cr 79% Fe ALLOY, ANNEALED O. 18NillOr-7l% Fe ALLOY, ANNEALED 1 Creep, (luring stress-aging, reported as inches per inch length.

fiThree alloys of this type werestress-aged at temperatures below recrystallization, the alloys beingin the annealed state previous to stressaging. Whereas the copper-base alloys must be cold worked in order to sustain effective loads for stress-aging, such iron-base alloys as these are inherently strong enough in the annealed state to be very effectively stress-aged. This is mentioned in order to again emphasize the fact that cold work is not an inherent requirement for stress-aging, merely that cold work strengthens the alloy previous to stress-aging,,thereby percreased the proportional limit for this alloy from mitting more eflective subsequent stress-aging. 25,000 p. s. i. in the annealed state to 89,200 p. s.-i.,

Iii even higher than the tensile strength as annealed (85,000 p. s. i.).

The maximum stress-aging effects for these alloys are given respectively in lines 3, 9 and 16 (15 is more practical) of Table 8. These maximum stress-aged properties markedly exceed the properties for the same alloys when coldwvorked 50% although the stress-aging treatment involved no measurable plastic deformation. As is the "case with the copper-base, alloys, these Febase alloys can be similarly improved by stressaging subsequent to -cold-working.

CLASS D'.PRECIPITATION-HARDENED Fe-BASE ALLOY Ni-Span-C hasthe following composition: 42% Ni5.5Cr2.5Ti0.l carbonbal. Fe, containing usual minor impurities. Due to its titanium content (together with Ni), this alloy can be markedly-improved by an inherent precipitation reaction. Its complete precipitation heattreatment is as follows: (a) solution treatmam; heat to 955C. and quench, (b) then age -2 "to 4 *hours at '675 C. The precipitation-hardiningeffect'of aging the solution-treated alloy does not appreciably change the properties until the aging'temperature'exceeds 500 C. This permits stress a'ging tests of the solution-treated alloy in the range of l500 C. Without appreciable overlapping of inherent precipitation reaction. In Table 9, the properties of-the solution-treated alloy are given in line 1. Lines 2 and 3 show that this alloy yields stress-aging effects similar to those of the 3 annealed Fe-base alloys cited above (Class-Cl.

TABLE 9 Ill 20 essentially inert at temperatures below and about at the critical aging temperature.

Line 4 of Table 9 gives the tensile properties for Ni-Span C fully age-hardened. Lines 5, 6 and 7 show that these properties can be'markedly improved by subsequent stress-aging. The preferred stress-aging treatment is that of line 5.

cmssn soili'b SOLUTION iii-Basin ALLOYS Two non-heat-treatable (except forannealing orstrain relief treatments) Al-base alloysare known commercially-as 2S and 52S. Their compositions are as'followsz 2S.From 1 to 2% impurities, chiefly Fe and-Si. 52S.2.5% Mg-0.25% Cr-ordinary impurities.

Stronger lattice binding within 'the crystal structures of metals and alloys results in higher melting temperatures, since thegreater thermal agitation at higher 'temperatures is required to Properiies of Ni-Span C' precipitation-harden- 'ing alloy, in its solution treated and itsprecipitation-"a'ged conditions, efiect of stress-aging marke d decreases in strength properties.

Stress-Age Treatmm P 1 il t 'r 1 131 r o t rop se ensie 1 ong. er en Lme S Limit S Yicldh Strength Per Cent R. A.

tress, trengt p. S i Hrs. 0.

PLAIN SOLUTION-TREATED None 15, 000 1 35, 000 90, 000 f 35,000 '4' 400 0 30.000 51,000 03,000 38 00 35, 000 4 300 0 02, 200 34, 200 90, 300 39 00 PRECIPITATION-AGED Creep, during stress-aging, reported as inches per inch length.

Fo'r alloys that ca-n be *precipitation-hardened to maximum properties subsequent aging at temperatures below (and at) that of the critical h'eat-treatmentwill effect he further-increase in In fact, the only effect for *extremelylong periods of heating at such'temperatures will be a tendency to over-age with It is obviousthe'refore that, if stressaging is superimpos'ed-upo'n aprecipitation-hardened alloy, any improvement in strength properties will be due to stress aging effects in the matrix solid solution, the previously precipitated phase remaining 'tion) sults of stress agingtreatments, as given in Table 10 bear this 0111;, although it is to be noted that their proportional limits can be increased in the range of 20 to'30% (lines 2, 3, and 9, 11 of TablelO.

TABLE 10 Effect of stress-aging on two cold worked, solid solution, :alumtnwm alloys Stress-Age Treatment Line Creep, Bropfl Tensile Elong,

Stress Hrs o Per Cent Limit Strength Strength P Cent p. s. i.

GOLD WO RKED ill-ALLOY 2S V; H

1 None 1 11, 000 17, 850 20, 000 20 2 11, 000 4 100 0 13, 600 17, 700 19, 900 19 3 000 4 125 0 13, 600 17, 700 19, 600 21 4 10, 500 4 150 0 12, 400 17, 600 19, 500 19 5 7, 750 4 175 0 12, 900 17, 350 19, 200 19 6 7, 750 4 200 3 12, 000 14, 900 17, 600 21 COLD WORKED Al-ALLOY 52S H None 1-7, 500 26, 000 32, 800 16 As the heat-treated Al-alloys possess considerably higher strength properties, improvements of 20 to in their elastic properties would be more valuable from a commercial standpoint.

cLAss F.HEAT-TREATED Al-BASE ALLOYS For alloys that are age-hardened to maximum properties, subsequent aging at temperatures below (and at) that of the original aging treatmentwill effect no further increase in strength properties. In fact, the only efiect for even extremely long periods of heating at such temperatures will be a tendency to over-age withmarked decreases in strength properties. It is obvious that,; if i stress-aging is superimposed upon a precipitation-hardened alloy, any increase in strength properties will be due to stress-aging effectsiin the matrix solidsolution, the quantity of the previously precipitated phase remaining unaffected but added to by further precipitation effects induced by the new stress-aging treatment. Thus the matrix (parent) solid solution phase of the fully precipitation-hardened alloys shouldbe susceptible to subsequentstress-aging treatment just as in the case of unheat-treatable'solid-solution alloys (alpha brass for example). However, since the matrix solid-solutionphase of fully age-hardenedalloys has already been effectively depleted of solute metal content by the normal precipitation reaction, the eifectgof subsequent stress-aging on properties yet considerably lower than for such alloys as alpha brasses and bronzes. Also, the heat-treatable Al alloys will withstand much higher load stresses during stress-aging than do the solid solution Al alloys 2S and 28.

Each of the 5 representative heat-treatable A1 alloys was given its standard complete heattreatment for maximum known strength properties. Samples of each heat-treated alloy were then stress-aged at stresses including values equal to the room temperature proportional limits and 0.2% offset yield strengths for the specific (heattreated) alloy for 4 hrs. at each of a succession of temperatures ranging from 100 C. to above the characteristic aging temperature for each specific alloy. The standard period of 4 hours for aging under the stated applied loads was arbitrarily chosen since it has been determined that 15 minutes of stress-aging generally yields from 50 to of the effect of 4 hours stress-aging and 4 hours aging generally yields or more of the effect of 20 hours of stress-aging. In industry, 4 hours for stress-aging would appear to be about the maximum practical time that would be allowed for this treatment and, for maximum economic efficiency, a 15 minute stress-aging period should be used whenever it is sufiiciently efiicacious.

heat-treatable Al-alloys follow:

TABLE 11 Properties of aluminum alloy 148T after stress-aging ,Stress-Age Treatment O 27 off Creep, Propl 0 Tensile Elong. Line set Yield Y sues-s, Hrs O 0 Per Cent Limit Strength Strength Per Cent p. s. i

23 Line 1 gives the tensile properties for alloy 148T in its normally age-hardened condition. Line 3 gives the preferred stress-aging treatment for maximum elastic (engineeringly useful) prop- Line 1 gives the tensile properties for alloy ITS-T in its normally heat-treated condition. Line 10 gives the preferred stress-aging treatment for maximum elastic properties Without deerties without destructive (practically measurstructive creep. By line stress-aging treatable) creep. By line 3 stress-aging treatment, merit, the proportional limit is increased by h p p i limit is in r y about 20% and the 0.2% ofiset yield strength i increased with no lossin elon at o Plain p onged ag g by 20%. Even though the elongation for this at (1n absence of y appllsd Stress) treatment decreased to 13% this value still represplts m ovelafgmg (or anneaihng) l 5 10 resents excellent ductility for en ineering use. ertles as QW 281000 proportlonal 3 900 An alternative beneficial stress-aging treatment 0.2% offset yield strength, 43,000 tensile strength is that of line 5. It W111 be noted that fol alloy and 16% elongatlon' These propelmes are to be 178T (and several others of thi rou stresscompared with line 6 for 150 C. stress-aging. g p Plain prolonged aging at 5 results in the 15 aging under the proportional limit stress with infollowing properties: no proportional limit, 8:009 creasing stress-aging temperature causes initial 0.2 offset yield strength, 10,000 tensile strength Increases 1n elastic propertles; then passes and elongatjon These properties Should b through a minimum value to contlnue to a second compared with the 250 C. stress-aging of line 10. and considerably greater maximum effect.

TABLE 13 Properties 0 aluminum alloy 185T after stress-aging Stress-Age Treatment 0 2 on Cree Pro '1 Te ile Elo Lme Stress l' llt Lin it g g i g Strc n gth Per (a ent s. Hrs. c. g

None 30,050 47, 500 50,100 12 30,000 4 0 39,100 51,000 02,200 10 47,500 4 100 0 45,800 51,300 50,200 14 ,000 4 0 39,000 50,700 01,200 12 47,500 4 125 0 45,700 52,800 02,200 12 30,000 4 0 33,000 49,000 59,500 12 47,500 4 150 2 45,200 52,700 58,500 12 30,000 4 0 33, 300 40,100 00, 000 12 ,000 4 200 0 30,200 47, 300 58,800 10 ,s00 4 225 0 31,000 45,800 54,500 12 23,000 4 250 0 30,200 42,200 50,100 10 These comparisons show that, not only does Line 1 gives the tensile properties for alloy Stress-2mg Yleld Improved Propertles but that 45 T in its normally heat-treated condition.

stress-aging opposes the normal overaging or annealing reaction until the temperature is high enough (in excess of the normal critical aging temperature of the standard heat-treatment) for the normal annealing (over-aging) tendency to completely overcome the stress-induced (stress) aging reaction. Such plain-aging vs. stress-aging comparisons hold for all 5 of these representative heat treated Al-alloys.

Lines 3 and 5 give two, practically equivalent,

preferred stress-aging treatments for maximum elastic properties (unaccompanied by destructive creep). The proportional limit is increased by about 30%, and the yield strength by about 8% with elongation constant or slightly increased.

TABLE 12 Properties of aluminum alloy 1 7ST after stressaging Stress-Age Treatment 0 7 Ofi- Creep Propl 0 Tensile E10ng.. Linc set Yield Sues? Hrs u 0 P01 Gent Limit Strength Strength Pei Cent p. s. i

TABLE 14:

Stress-Age Treatment 7 Ofi v .2 0 El Line Oregp, Prop 1 Yield Tensile ong.,

SW5? H o 0 Per (,ent Limit Strength Strength Per Cent p. s. 1.

None 38, 000 49, 750 69, 300 17 38, 000 4 100 0 40, 400 50, 300 71, 800 17 49, 500 4 100 0 48, 700 51, 300 70. 000 38, 000 4 125 0 41, 000 49, 300 69, 800 19 TABLE 15 7 Properties of aluminum alloy 758T after stress-aging to stress age to at least double their proportional or elastic limits. .Lesser savings apply to those alloys of smaller inherent stress-ageability.

What I claim as new and desire to secure by Letters Patent of the United States is 1. The method for improving the properties 0 an alloy consisting wholly or in major part 'of one or more solid solution phases which comprises artificially aging said alloy for from 15 minutes to 20 hours at a temperature below but within 100 C. of the recrystallization tempera- Stress-Agc Treatment 0 27 Off Line Creep, Propl St {field Tensile Elong,

Sues? H-rS O C Pei Cent Limit Strength Strength Per Cent p. s. l.

None 57, 700 71, 500 82, 000 11 58, 000 4 100 0 61, 600 71, 900 800 14 as, 000 4 125 0 so, 000 72, 200 82,000 13 58, 000 4 150 0' 65. S00 73, 700 81, 000 ll 50, 200 4 175 1 61, 000 70, 500 77, 000 10 35, 000 4 200 0 48, 900 58, 700 68, 500 7 25, 000 4 225 0. 5 35, 700 42, 300 54, 000 5 Line 1, Table 15, gives the tensile properties for this alloy in its normally heat-treated condition. Line 4 gives the preferred stress-aging treatment for maximum elastic properties. The proportional limit is increased by about 16% with no loss in elongation. i

The true explanation for the remarkable results of the new stress-aging treatment has not been wholly or definitely determined. A considerable change in crystal structure seems necessarily involved but it could. well be submicroscopic. It has been observed that cold worked structures stress-aged take five to ten times as long to etch and give very sharp structures for clear focussing whereas plain cold worked structures are impossible of sharp focussing at magnifications as high as 2000 diameters. Variations in structure, such as segmentation of slip bands and apparent precipitation have been observed but not proved. A lattice ordering reaction may be involved in some instances.

One advantage of my improved stress-aging process is that present operational stresses can be borne by half the present cross sections or even less if the operating parts are stress-aged and also that present operating parts can support twice the present operational stress-or even more if the partsare stress-aged. This 2:1 ratio applies for the many alloys that have the capacity ture and under an externally applied stress at least equal to the proportional limit but not exceeding the 0.2% offset yield strength of said alloy prior to said aging. V

2. The method for improving the properties of an alloy consisting principally of one" or more solid solution phases which comprises artificially aging said alloy at a temperature below but within C. of the recrystallization temperature forat least 15 minutes under an externally applied stress between the proportional limit and 0.2% ofiset yield strength of said alloy prior to' said aging.

3. The method for improving the properties of an alloy consisting principally of one-or more.

solid solution phases-which comprises artificially aging said alloy at a temperature belowbut within 100 C. of the recrystallization temperature for from 4 to 20 hours under an externally applied stress which is at least equal to the proportional limit and does not exceed the 022% offset yield strength of said alloy prior to said aging.

at. The method for improving the properties of an alloy consisting principally of one or more solid solution phases, which comprises aging said alloy at a temperature below but Within about 100 C. of the recrystallization temperature of said alloy for from 15 minutes to 4 hours and under an externally applied stress which is at least equal to the proportional limit of said alloy prior to said aging and which is insufficient to cause a permanent deformation of more than one mil per inch.

5. The method for improving the properties of an alloy consisting principally of one or more solid solution phases which comprises aging said alloy for one-quarter to twenty hours at a temperature within the range of f om 100 C. below the recrystallization temperature up to the recrystallization temperature of said alloy and under an externally applied stress at least equal to the proportional limit or" said alloy and not exceeding the 0.2% ofiset yield strength of the alloy prior to said aging, the time, temperature and stress conditions being such that no praotically measurable permanent deformation of the alloy is efieoted.

6. The method for improving the properties of alrralloy-consistingprincipally of one or more at a temperature below but within about 160 C.

of the recrystallization temperature of said alloy, said stress being within the range between the proportional limit of said alloy and its 0.2% offset yield strength determined for the alloy in the condition preceding aging, and cooling said alloy to room temperature under said applied stress.

8. The method for improving the properties'of a copper base alloy consisting principally of one or more solid solution phases, which comprises artificially aging said alloy at a temperature of from 175 to 275 C. under an externally applied stress within the range of the proportional limit and the 0.2% offset yield strength as determined for said alloy in its condition preceding aging, said stress being applied to said alloy for from 15minutes to 4 hours.

9. The method for improving the properties of a; copper base alloy Wire consisting principally of one or more solid solution phases, which comprises applying an external stress to said wire for from 15 minutes to 4 hours while it is aged at a temperature below but within about 100 C. of. the recrystallization temperature of said alloy, said stress being within the range between the proportional limit of said alloy and its 0.2% offset yield strength as determined for the alloy in the condition preceding aging, the time, stress and temperature conditions being insufiicient to effect within the range of the proportional limit and the- 0.2% offset yield strength as determined for said alloy in its condition preceding aging, said stress being applied to said alloy for from 15 minutes to 4 hours.

12. The method for improving the properties of a stainless steel, which comprises applying an external stress of about 90,000'p. s. i. to said alloy. for from 15 minutes to 20 hours Whileitis aged at a temperature of about 400 C. and coolingsaid alloy to room temperatur under said applied;

stress.

13. The methodof claim 12 wherein the stress is applied for from 15 minutes to 4 hours.

14. The methodfor-im-proving the propertiesoi an aluminum base alloy consisting principally of one or more solid solution phases, whichcorm prises artificially aging said alloy ata tempera-.

ture of from 100 to 200 C. under an externally applied stress within the range of the proportional limit and the 0.2% offset yield strength as determined forsaid alloy in its condition precedingv aging, saidstressbeing applied to said alloy for from 15 minutes to 4 hours.

15. The method for improving the properties of a cold worked aluminum base alloy consisting principally of one or more solid solution phases, which comprises applying an external stress to said alloy for from 15 minutes to 20 hours while it is aged at a temperature below but within the range of from 100 C. below the recrystallization temperature up to the recrystallization temperature of said alloy, said stress being'within the range between the proportional limit of said alloy and its 0.2% offset yield strength as determined for the alloy in the condition preceding aging, and cooling said alloy to room temperature under said applied stress.

RICHARDS H. HARRINGTON.

REFERENCES CITED The following references are of record in thefile of this. patent:

UNITEDSTATES PATENTS Number Name Date 1,887,339 Stroble Nov. 8, 1932 1,942,025 Frost s Jan. 2, 1934 2,261,878 Hathaway Nov. 4, 1941 2,351,922 Burgwin June 20, 1944 

1. THE METHOD FOR IMPROVING THE PROPERTIES OF AN ALLOY CONSISTING WHOLLY OR IN MAJOR PART OF ONE OR MORE SOLID SOLUTION PHASES WHICH COMPRISES ARTIFICIALLY AGING SAID ALLOY FOR FROM 15 MINUTES TO 20 HOURS AT A TEMPERATURE BELOW BUT WITHIN 100* C. OF THE RECRYSTALLIZATION TEMPERATURE AND UNDER AN EXTERNALLY APPLIED STRESS AT LEAST EQUAL TO THE PROPORTIONAL LIMIT BUT NOT EXCEEDING THE 0.2% OFFSET YIELD STRENGTH OF SAID ALLOY PRIOR TO SAID AGING. 